The platelet is an important component in the pathophysiology of many diseases. The interaction of human platelets with human antibodies will be studied in vitro and in vivo. Both intact platelets and platelet membranes will be employed. Platelet antigens will be determined and the antigen-antibody affinities measured, as well as their alteration by vincristine and corticosteroids. Platelet antigen mobility will be assessed by electron microscopic studies. The clinical course of patients will be correlated with the extent of platelet associated IgG, C3b, C3d, platelet functional alteration, antibody affinity and the presence of platelet-associated viral antigen. The development of a new assay to rapidly and quantitatively assess patient platelet-associated IgG and C3 makes these studies now tenable. These studies should help to more effectively define immune platelet disorders and assess both in vitro and in vivo their pharmacologic modification. They should help determine which platelet-antibody-complement interactions are most amenable to therapy.